Extended commercial services of third generation mobile communication represented by HSDPA/HSUPA have been started and multimedia communication such as data communication and video communication has been increasing in popularity. Therefore, data sizes are expected to increase even more in the future, and growing demands for higher data rates are also anticipated. Then, technical studies are underway to realize a higher data rate of maximum downlink transmission rate of 100 Mbps.
To realize this higher data rate, as a rate matching technique of forming a transmission bit sequence of a desired coding rate or a desired coding length from a bit sequence subject to error correction coding (encoded bit sequence) with a given minimum coding rate (a mother rate), rate matching using a circular buffer (circular buffer-based rate matching) is proposed. By making a buffer to which an encoded bit sequence is written a circular (cyclic)-structure buffer, the addresses upon forming a transmission bit sequence of a desired coding rate or a desired coding length are easily managed, and complexity upon installing a buffer is reduced. Further, with a circular buffer-based rate matching, rate matching is performed on a per divided code block basis, so that it is possible to process a plurality of code blocks in parallel.
Furthermore, studies are conducted for HARQ (Hybrid Automatic Repeat reQuest) control used by combining circular buffer-based rate matching (see Non-Patent Document 1). With this HARQ control, as shown in FIG. 1, a circular buffer to which encoded bit sequences are written is divided into a plurality of buffers such that each head address of a divided buffer are associated with each RV (Redundancy Version) index in HARQ. Then, a transmission bit sequence fulfilling a desired coding rate (or a desired coding length) is consecutively read from a read start address (i.e. starting position of reading) calculated based on the selected RV index.
However, the coding rate or the coding length of an encoded bit sequence is usually determined based on a CQI (Channel Quality Indicator), and therefore changes over time. Further, the buffer size of the divided buffers (i.e. divided buffer size) is preset and does not change. Therefore, a transmission bit sequence and the divided buffer size rarely match. Accordingly, as shown in FIG. 1, a gap or overlap is produced between transmission RVs, to deteriorate decoding performance on the receiving side. In the example shown in FIG. 1, a gap is produced between the first transmission RV and a second transmission RV and between the second transmission RV and a third transmission RV, and an overlap is produced between the third transmission RV and the first transmission RV.
To prevent gaps and overlaps between RVs, studies are conducted for HARQ control that defines a read direction (a forward direction or backward direction) from a circular buffer, in addition to a read start address from a circular buffer. The number of conventional RVs has been four, taking into account the tradeoff between an overhead and rate matching performance, and therefore two control information bits have been conventionally prepared for reporting the RV configuration to the receiving side. Then, with the technique disclosed in Non-Patent Document 2, among two control information bits, one bit is used for information about a read start address and the other bit is used for information about the read direction. Consequently, according to Non-Patent Document 2, two read start addresses, a read start address as RV=0 and read start address as RV=1, are defined. Then, Non-Patent Document 2 discloses a concrete example, assume that both the first transmission RV and a second transmission RV are RV=0, upon the first transmission, reading a transmission bit sequence in the forward direction from the read start address, RV=0, and upon a second transmission, reading a transmission bit sequence in the backward direction from the read start address, RV=0.    Non-Patent Document 1: R1-072604, “Way forward on HARQ rate matching for LTE,” 3GPP TSG RAN WG1 #49, Kobe, Japan, May 7-11, 2007    Non-Patent Document 2: R1-073563, “Redundancy version definition for DL-SCH,” 3GPP TSG RAN WG1 #50, Athens, Aug. 20-24, 2007